Top-pumped waveguide amplifier

ABSTRACT

The present invention relates to a waveguide amplifier which is comprised of silica or silica-related material co-doped with silicon nanoclusters and rare earth elements, and more particularly, to a waveguide amplifier with higher efficiency enhanced by top-pumping method and focusing means for pumping light. The waveguide amplifier of the present invention comprises of: (a) a substrate; (b) an optical waveguide including: a lower cladding layer formed on the substrate; a core layer which is made of silica or silica-related material co-doped with silicon nanoclusters and rare earth elements on the lower cladding layer and has a refractive index higher than that of the lower cladding; and an upper cladding layer formed on the core layer; and (c) a light source, spaced apart from the waveguide, for optically pumping the waveguide, wherein the waveguide amplifier operates by exciting the rare earth elements through electron-hole combinations in the silicon nanoclusters.

TECHNICAL FIELD

The present invention relates to a silica or silica-based waveguideamplifier which is co-doped with silicon nanoclusters and rare earthatoms, and more particularly to a waveguide amplifier with improvedefficiency, resulting from using a top pumping process and means forfocusing a pump light.

BACKGROUND ART

Recently, rare earth-doped silica or silica-based materials have mostwidely been used to manufacture waveguide amplifiers. However, because alight source directly pumps the rare earth atoms, and because rare earthatoms have narrow absorption bands and small absorption cross sectionsof the order of 4×10⁻²¹cm², in order to pump such a waveguide amplifier,an end fire technique must be used for coupling light into the waveguidethrough an optical fiber using a high-priced laser as the light source.

In case of doping silicon nanoclusters with rare earth atoms in such awaveguide, an electron-hole combination formed in the nanocluster cangive rise to excitation of the rare earth atoms. The effectiveexcitation cross section is approximately 1×10⁻¹⁵cm². Considering thatthe concentration of the silicon nanocluster is generally 1×10¹⁸ cm³,the above value corresponds to an absorption depth of 10 μm or less. Inthis regard, if a waveguide amplifier made of the aforementioned twomaterials is subjected to a top pumping process, in which a light sourceis positioned above the waveguide, although no such attempts have beenmade, it may have an enhanced efficiency. In particular, because thereare no particular limitations to a light source so long as the lightsource can generate carriers in the silicon nanocluster, a low-pricedwide band light source such as LED can be used instead of a high-pricedlaser. However, contrary to the end fire technique for coupling almostall light from a light source into a waveguide through an optical fiber,the top pumping process, in which a light source is positioned above awaveguide, cannot allow all light from the light source to enter thewaveguide. As a result, an actual pump power is substantially decreasedrelative to that from the light source, thereby lowering the excitationefficiency.

DISCLOSURE OF THE INVENTION

Therefore, the present invention has been made in view of the aboveproblems, and it is the objective of the present invention to provide awaveguide amplifier using a top pumping process, which can efficientlyfocus light from a light source onto the waveguide.

In accordance with the present invention, the above objective and otherobjectives can be accomplished by the provision of a waveguideamplifier, comprising:

(a) a substrate;

(b) an optical waveguide including a lower cladding layer formed on thesubstrate, a core layer formed on the lower cladding layer, which ismade of a silica or silica-based material co-doped with siliconnanoclusters and rare earth atoms and has a higher refractive index thanthe lower cladding layer, and an upper cladding layer formed on the corelayer; and

(c) a light source for pumping the waveguide, which is positioned abovethe waveguide, characterized in that the waveguide amplifier is operatedin a manner such that the rare earth element is excited through anelectron-hole combination formed in the silicon nanocluster.

Preferably, the light source is a visible light source, more preferably,LED or a flash lamp.

Preferably, multiple optical waveguides are arranged in the beam spot ofthe light source.

Preferably, the waveguide amplifier further comprises means for focusinga pump light from the light source onto the waveguide.

In accordance with the first embodiment of the present invention, thepump light from the light source is refractively focused onto thewaveguide, which is formed with prominences in the form of convex lenseson the upper cladding layer thereof.

In accordance with the second embodiment of the present invention, thepump light from the light source is reflectively focused onto thewaveguide, which is formed with mirror surfaces.

In accordance with the third embodiment of the present invention, thepump light from the light source is reflectively focused onto thewaveguide using mirror surfaces formed on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objectives, features, and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view showing a top pumping process;

FIG. 2 is a graph showing intensities of output light signals in awaveguide amplifier using a top pumping process;

FIG. 3 is a schematic view showing a top pumping process in waveguidessharing a pump light;

FIG. 4 is a schematic cross sectional view showing a waveguide amplifierusing the optical focusing technique according to the first embodimentof the present invention;

FIG. 5 is a schematic cross sectional view showing a waveguide amplifierusing the optical focusing technique according to the second embodimentof the present invention; and

FIG. 6 is a schematic cross sectional view showing a waveguide amplifierusing the optical focusing technique according to the third embodimentof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the embodiments of the present invention will be describedin detail with reference to the accompanying figures.

FIG. 1 is a schematic view showing a top pumping process. A silica orsilica-based waveguide which is co-doped with silicon nanoclusters andrare earth atoms strongly absorbs visible light but does not absorbinfrared light. Based on this fact, a wide band light source (not shown)is positioned above the waveguide 100 in a manner such that it transmitsvisible pump light to the waveguide. The pump light injected into thewaveguide 100 generates electron-hole combinations in the nanocluster,whereby the rare earth element is excited. An input light signal isamplified through the waveguide 100 using the energy generated from theexcited rare earth element and then exits in the form of an output lightsignal.

FIG. 2 is a graph showing intensities of output light signals in awaveguide amplifier using a top pumping process. The graph curvesrepresent the spectra of output light signals after inputting lightsignals into the waveguide and pumping the waveguide using a pump light.Referring to FIG. 2, it can be seen that input light signals areamplified at a pump power of more than 0.5 W/cm². The embodiments ofFIG. 2 did not use means for focusing a pump light onto the waveguide.The use of the focusing means can further increase the amplificationefficiency of the waveguide amplifier.

Meanwhile, in case of such a top pumping process using a wide band lightsource, because light from the light source is scattered, all the lightfrom the light source is not used to pump the waveguide amplifier.Therefore, in order to solve this problem, there is a need to increasethe size of the waveguide, or alternatively to focus light from thelight source onto the waveguide.

However, the size of the waveguide cannot exceed 10 μm because of anoptical fiber-to-waveguide coupling problem and the like. Furthermore,for the purpose of focusing light from the light source, it isimpossible to infinitely reduce the beam spot size. Conventionally, thebeam spot size in a wide band light source is approximately 1 mm. Inthis regard, it is possible to arrange several tens of waveguides in thebeam spot. As a result, the pumping efficiency can be increasedcorrespondingly. FIG. 3 is a schematic view showing a top pumpingprocess in waveguides sharing a pump light. Referring to FIG. 3,multiple waveguides 100 are formed in parallel with each other on asubstrate 300. These waveguides 100 share the beam spot of light from awide band light source 310, which is positioned above the waveguides.

Meanwhile, the waveguide amplifier of the present invention furthercomprises means for focusing light from a light source onto thewaveguide in the top pumping process. Such focusing means will bedescribed in the following embodiments. The same constitutional elementsare indicated with the same reference numbers and thus repetitivedetailed descriptions thereof will be omitted.

FIRST EMBODIMENT

It is understood that in order to increase the pumping efficiency, aftercollecting light from a light source using an optical device such as alens, a waveguide can be pumped using the light so collected. However,because the size of the waveguide is very small and costly, there is alimitation to the use of such a separate optical device. Therefore, inorder to collect light at a low price, prominences 410 are formed on anupper cladding layer 400 of core layers 102 to act as convex lenses, asshown in FIG. 4. The prominences 410 are formed in a linear arrangement,similar to the core layers 102. They may be formed by etching the uppercladding layer 400, or by using a polymer such as PMMA subsequent tovapor deposition of the upper cladding layer 400.

In case of etching the upper cladding layer 400 for the purpose offorming the prominences 400, a “diffusion-limited etching process” canbe used. Although such an etching process is little used inmanufacturing of semiconductor devices, it is often selectively used toetch structures requiring special shapes. The principle of the etchingdepends on diffusion of an etching reagent. The etching rate increasesat an area where sufficient etching reagent is provided due to rapiddiffusion; but the etching rate decreases at an area where insufficientetching reagent is provided due to slow diffusion. In this regard, anetching mask with an opening line pattern can be used. An etching rateis slow at the center portion between the opening lines and is fast atthe opening lines. As a result, linear prominences 410 are formed.

In FIG. 4, reference number 430 indicates a substrate and referencenumber 420 indicates a silica lower cladding layer.

SECOND EMBODIMENT

FIG. 5 is a schematic cross sectional view showing a waveguide amplifierusing the optical focusing technique according to the second embodimentof the present invention. According to the second embodiment of thepresent invention, a pump light is focused onto a core layer 102 usingthe reflection of a concave mirror M1 which is formed by deeply etchingand then coating a lower cladding layer 422. In this case, the positionof the core layer 102 and the curvature of the concave mirror M1 aremodulated in order for the core layer 102 to be positioned at the focalpoint of the concave mirror M1. In FIG. 5, reference number 412indicates an upper cladding layer.

THIRD EMBODIMENT

In the second embodiment, a deep etching process was used to form amirror surface. In the case that it is somewhat difficult to carry outsuch a process, it is preferable to use a mirror without a deeply etchedstructure. FIG. 6 is a schematic cross sectional view showing awaveguide amplifier using the optical focusing technique according tothe third embodiment of the present invention. Reference is made to FIG.6, first, grooves in the form of line are formed in a transparentsubstrate 432. Then, the grooves are mirror coated to thereby formmirrors M2. Similar to the second embodiment, the core layer 102 ispositioned at the focal point of each mirror M2.

INDUSTRIAL APPLICABILITY

As apparent from the above description, the present invention provides awaveguide amplifier with low cost and high efficiency, resulting fromincreasing the pump light efficiency per waveguide using a wide bandlight source. This enables integration of optical devices as well aslowering unit cost of the optical device. As a result, remarkabledevelopment is accomplished in the field of optical devices.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A waveguide amplifier, comprising: (a) a substrate; (b) an opticalwaveguide including a lower cladding layer formed on the substrate, acore layer formed on the lower cladding layer, which is made of a silicaor silica-based material co-doped with silicon nanoclusters and rareearth atoms and has a higher refractive index than the lower claddinglayer, and an upper cladding layer formed on the core layer; and (c) alight source for pumping the waveguide, which is positioned above thewaveguide, characterized in that the waveguide amplifier is operated ina manner such that the rare earth element is excited through anelectron-hole combination formed in the silicon nanocluster.
 2. Thewaveguide amplifier as set forth in claim 1, wherein the light source isa visible light source.
 3. The waveguide amplifier as set forth in claim2, wherein the light source is LED or a flash lamp.
 4. The waveguideamplifier as set forth in claim 1, wherein multiple optical waveguidesare arranged in the beam spot of the light source.
 5. The waveguideamplifier as set forth in claim 1, further comprising means for focusinga pump light from the light source onto the waveguide.
 6. The waveguideamplifier as set forth in claim 5, wherein the pump light from the lightsource is refractively focused onto the waveguide, which is formed withprominences in the form of convex lenses on the upper cladding layerthereof.
 7. The waveguide amplifier as set forth in claim 5, wherein thepump light from the light source is reflectively focused onto thewaveguide, which is formed with mirror surfaces.
 8. The waveguideamplifier as set forth in claim 5, wherein the pump light from the lightsource is reflectively focused onto the waveguide using mirror surfacesformed on the substrate.